Liver Chemistry Tests - AST, ALT, Alkaline Phosphatase and Bilirubin Metabolism

AST and ALT:

 • AST: liver > skeletal muscle, cardiac muscle > kidney, brain. In liver, 80% of AST is in mitochondria (mAST), Rest cytoplasmic (CAST)

 • ALT: primarily within liver

Alkaline phosphatase:

 • Synthesis: in hepatocyte canalicular membrane (NOT within bile duct cells)

 • Other locations: bone, intestine, placenta

 • Increased levels: due to increased synthesis, not reduced clearance

    - AP synthesis stimulated by bile acid accumulation in hepatocyte

    - Elevation usually occurs several days after initial insult

Bilirubin metabolism:

 • Breakdown of heme (80% Hgb, rest myoglobin, cytochromes)

 • Unconjugated circulates with albumin in serum

 • Released from albumin upon hepatocyte entry

 • UGT1A1 responsible for conjugation in hepatocyte

 • MRP2 transports conjugated bilirubin into bile canaliculi (ATP-dependent process)

 • MRP3 transports back into plasma where most undergoes reuptake into hepatocyte via OATP1B1 + B3



- Dr. Hersh Shroff @HershShroff



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